1. Field of the Invention
The present invention generally relates to heat removal from an electronic device, more specifically, the present invention relates to at least one embodiment in which heat is removed from a Light Emitting Diode (LED), or a plurality of LEDs, by way of a thermal connection with a heat absorbing material which removes heat from the LED by undergoing an endothermic reaction.
2. Background Art
LED light sources are well known in the art. LEDs are light sources based upon a semiconductor structure, specifically a diode structure, which emit incoherent light (which may be in the ultraviolet, visible, or infrared spectrum) when electrical current is passed through the semiconductor junction. The original uses of LED light sources were in low-power applications such as indicator lights on instrumentation panels and the like. However, recent developments in LED technology have increased the output power and efficiency of LED sources so that it is now feasible to utilize them in traditional lighting applications previously reserved for incandescent, fluorescent, sodium, and similar lighting technologies. Commercially available LED light sources surpassed incandescent light source in terms of efficiency in or around 2002. More recently, LED light sources have become commercially available which exceed fluorescent light sources in efficiency. Fluorescent light sources typically exhibit around 75-100 lumens per Watt (lm/W) efficiency; however LED light sources have been recently introduced into the lab which exhibit 130 lm/W, and there are other LED light sources available and currently in development which exhibit even greater efficiency.
As the output power of the commercially available LED light sources has continued to improve, it has become necessary to develop methodologies and structures for removing the heat generated by the LED from the LED semiconductor junction. Typical problems caused by heating of the semiconductor junction and surrounding structure are: 1) failures brought on by such occurrences as non-homogenous distribution of the current density over the junction (“current crowding”), which causes a local hot spot in the diode junction leading to early failure due to thermal runaway; 2) nucleation and growth of dislocations in the active region of the diode in which the radiative recombination occurs due to the existence of an existing defect in the semiconductor crystalline structure and which is accelerated by heat; 3) degradation of materials utilized in the LED, such as phosphor or encapsulate, causing loss of efficiency and changes in output color; and 4) electromigration of metal atoms at the metallization layers of the diode causing growth of conductive “whiskers” and early failure. This is not a complete list of the undesired effects brought on by elevated temperatures; it is provided herein simply as a list of exemplary effects.
As an example of an LED that may be used in illumination see the LUXEON® products from Philips. The LUXEON® product line data sheets provide information for these products at 25°, or approximately room temperature. It is therefore desirable to operate these products and those like them as close to room temperature as possible. A heat conduction/dissipation apparatus is therefore required in such applications in order to remove heat from the LED in an effort to operate the LED as close to the data sheet temperature as practicable.
Attempts have been made to provide structure for removing the heat from the semiconductor junction of an LED. See, for example, U.S. Pat. No. 6,639,356 B2 (disclosing a heat dissipating base for an LED, and which further states the heat problem created by the use of LEDs in various applications, namely that “ . . . Heat generated by LED (sic) becomes the largest drawback in LED manufacturing industry, which causes different malfunctions to the circuitry.” U.S. Pat. No. 6,639,356 B2, col. 1. The invention of U.S. Pat. No. 6,639,356 B2 is an attempt to better conduct the heat away from the LED semiconductor junction so as to overcome the problems created by an overheated junction; namely reduced lifetime and output power. However, the invention of U.S. Pat. No. 6,639,356 B2 is directed to a package LED and simply providing a thermally conductive path out of the LED package. It does nothing to address the larger issue of dissipating heat away from the package.
There are a variety of means which have been employed to dissipate heat from packaged electronic devices. These means have met with limited success. One such means is simply to allow the heat from an electronic device to passively radiate from the electronic device package. This method allows the heat generated by the electronic device to radiate to the surrounding environment. This is not an efficient means of heat removal. In the case of LEDs, as the LED's capability to output light has increased over time, the power consumed by the LED has also increased. The increase in power consumed by the LED results in a corresponding increase in waste heat generated by the LED. Passive cooling of a high powered LED does not remove a sufficient amount of heat to enable the LED to operate as efficiently as possible. In order to facilitate optimum LED performance, more heat must be removed from a high power LED than can be removed by conventional passive methods.
An improvement to mere passive radiation is the use of a thermal grease or similar thermally conductive material. Such a thermally conductive material is used to enhance the thermal connection between the electronic device and the surface upon which the electronic device is mounted. See, for example U.S. Pat. No. 6,444,496 B1 (disclosing that “thermal paste is often used as a high thermal conductivity interface material to fill the gaps between the back-side of chips, such as, flip chips, and the inside surfaces of the caps or heat spreaders.”). The use of such a thermally conductive material allows the heat from the electronic device to more readily radiate away from the electronic device and to be dissipated by the mounting surface. The use of a thermally conductive material to couple the electronic device to the mounting surface increases the ability of the electronic device package to radiate heat. However, in the case of high power LEDs, this method may not adequately remove the amount of heat necessary to achieve optimum LED performance.
Another passive means of providing for heat removal comprises mounting the electronic device on a circuit board which is designed to enhance thermal conductivity. See, for example, U.S. Pat. No. 6,201,701 (disclosing “[a]n integrated substrate comprising a heatsink base, and a multilayer circuit board bonded thereto, the multilayer circuit board having the capability to interconnect both power and control semiconductor elements and efficiently dissipate heat therefrom.”). This method may dissipate more heat when compared to passive radiation of the electronic device package alone; however, the heat dissipation achieved by using circuit boards with enhanced thermal conductivity may still not be sufficient to enable the electronic device to achieve optimum performance. This is particularly true of high power LEDs.
Still another passive means of providing for heat removal from an electronic device is by use of a conventional heat sink. A conventional heat sink is typically constructed from a thermally conductive material which has a large surface area. See, for example, U.S. Pat. No. 6,441,943 B1 (disclosing “[a ] passive heat sink including . . . a plurality of fins . . . . The fins have a large surface area to dissipate heat, which increases dissipation of heat from the LED lamp, thereby increasing the amount of power that can be applied to the LED lamp without damaging the LED lamp, which in turn increases the amount of light that the LED lamp can produce.”) While conventional heat sinks allow for improved cooling when compared to cooling the electronic device without such a device, the cooling achieved by means of a conventional heat sink may not be adequate to allow the electronic device to achieve optimum performance, particularly when a high powered LED is utilized. Additionally, the increased bulk of the design added by a conventional heat sink because of the need to maximize the surface area of the conventional heat sink may be undesirable.
Alternative methods of heat removal that have been employed include active heat removal solutions. One such active heat removal solution involves the use of a conventional electrical direct current fan. See, for example U.S. Pat. No. 6,270,313 B1 (disclosing “a fan that is most commonly employed in cooling the electronic devices. The directions of the inlet and the outlet of the airflow are arranged in parallel. The blades of the fan are rotated along an axis perpendicular to the plane of the blades and in parallel to the airflow direction. This configuration produces a direct pressure difference between the inlet and outlet of the airflow thus provides large airflow for cooling and removing the heat generated from the electronic circuits.”). There are multiple significant problems involved with using a conventional electrical direct current fan to cool an electronic device. The conventional electrical direct current fan may add an unwanted amount of bulk to the design. Additionally, the use of a conventional electrical direct current fan creates audible noise which may be undesirable to the electronic device application. Furthermore, the use of a fan to actively cool an electronic device may still not cool the electronic device sufficiently to achieve optimum performance of the electronic device. This is especially true in the case of high powered LEDs. Finally, the use of a fan may be undesirable because the fan must consume power to operate. This power consumption may reduce or even eliminate the power consumption benefits achieved by the use of high efficiency electronic devices, such as LEDs.
Another active method of heat removal which has been used is a piezo fan. See, for example, U.S. Pat. No. 4,780,062 (disclosing “[a] piezoelectric fan includes a tuning-fork type vibrator having two sides. A first piezoelectric element is formed on each of the two sides of the vibrator. Further, an exciting electrode is formed on each of these first piezoelectric elements. Also, a feedback electrode is formed on one of the first piezoelectric elements an electrically insulated from the exciting electrode. Furthermore, a blade is attached to the tip of each of the two sides of the vibrator. Then, this piezoelectric fan is driven in a self-exciting fashion by feeding an output from the feedback electrode to the exciting electrode.”). However, a piezo fan must consume power in order to provide any cooling benefit to the electronic device. The use of a piezo fan may be undesirable because of this necessary power consumption. Additionally, piezo fans may be undesirable because of the increased bulk which they add to a design and the cooling obtained through the use of these fans may not be sufficient, particularly when used with high power LEDs.
Another active method of heat removal involves the use of a Peltier element. See, for example, U.S. Pat. No. 4,238,759 (disclosing “[a] Peltier-cooled p-n junction in a monolithic solid-state electronic device embodying a substrate comprising a Peltier thermal surface layer disposed adjacent the active region of the p-n junction and in close proximity therewith, the Peltier thermal surface layer having an electrical conductor in circuit therewith affording an electrical path for the withdrawal or supply of Peltier effect current incident to the maintenance of the junction at a preselected temperature.”). Under certain limited conditions, a Peltier element may result in significant cooling of the electronic device. However, the Peltier element is extremely inefficient. The Peltier element requires the consumption of electricity to operate. In fact, the Peltier element may consume up to twice as much power as it dissipates. This increased power consumption is just one disadvantage to using a Peltier element. An additional disadvantage is that because of the inefficiency of the Peltier element, a significantly larger heat sink must be used to dissipate the additional heat created by the Peltier element. Furthermore, as the power transferred by the Peltier element increases, the temperature across the Peltier device tends to decrease; therefore, the use of a Peltier device may provide no cooling at all if the system in which it is used is high powered.
Another legacy means of active heat removal from an electronic device involves the use of a heat pipe. See, for example, U.S. Pat. No. 4,116,266 (disclosing “a closed container which is possessed of an input portion and an output portion for heat energy and is provided on the inside thereof with a wick extending throughout from the input portion to the output portion and disposed so that the resistance offered thereby to the flow of liquid gradually decreases from the input side to the output side. The heat medium which is vaporized on the input side is moved in the direction of the output side by virtue of the difference of pressure created inside the container. The heat medium which is deprived of heat and consequently liquefied on the output side is moved within the wick in the direction of the input side by virtue of capillary action coupled with the suction resulting from the vaporization of the heat medium in the input portion.”). The use of a heat pipe may provide a more efficient method of removing heat from an electronic device when compared to some passive methods; however, there are still drawbacks to this type of cooling system. The use of a heat pipe may be undesirable because the physical size of the heat pipe must be large enough to allow space for the material to vaporize and move away from the heat source. Additionally, the heat pipe may not remove as much heat from the electronic device as is required to achieve optimum performance, particularly in the case of high power LEDs.
Still another means of removing heat from an electronic device involves the use of a material which can only be used as a means of heat removal for a limited period of time. See, for example, U.S. Pat. No. 7,135,033 B2 (disclosing that “ice is employed as a phase change material for removing waste heat generated by alight source incorporated in a handpiece of a photocosmetic device.”). This may be an efficient means of heat removal until the material has exhausted its heat absorbing capabilities. At this point, the desirable thermal characteristics of the material are depleted and no further heat dissipation can be effected. In addition to the limited duration of such a solution, this solution is undesirable because it is not sustainable. That is, when this kind of a material has been depleted, it cannot automatically be renewed. A new amount of the heat dissipating material would have to be added to the system in order for the material to provide continuing heat dissipation functions.
Attempts have been made to address the problem of removing heat from electronic devices. See, for example, U.S. Pat. No. 6,202,739 B1 (disclosing a heat-dissipating apparatus which “includes a housing piece that includes a sealed chamber and a phase change material disposed within the sealed chamber. The heat-dissipating apparatus is thermally coupled to the heat-generating component and provides for heat dissipation by an endothermic process of changing phase from solid to liquid, thereby lowering the operating temperature of the heat-generating component.” U.S. Pat. No. 6,202,739 B1). However, the invention of U.S. Pat. No. 6,202,739 B1 does not address the use of phase change materials or other materials that may undergo endothermic reactions other than organic paraffin compounds, including n-eicosene, n-octadecan, and n-dotriacontane, low-temperature solder alloy, and a low-temperature metal, such as gallium.
See also, for example, U.S. Pat. No. 6,452,217 B1 (disclosing an LED lamp in which “phase change material is incorporated into the lamp to act as a source of heat removal.” U.S. Pat. No. 6,452,217 B1). However, the invention of U.S. Pat. No. 6,452,217 B1 discloses only the use of phase change materials, including waxes and eutectic salts, and discloses only that these materials are impregnated in a metal mesh or other metal structure. Furthermore, U.S. Pat. No. 6,452,217 B1 does not disclose the use of additional endothermic reactions, nor does it disclose the use of these materials in a form which is not embedded in a metal component of some sort.
See also, for example, U.S. Pat. No. 3,780,356 (disclosing “[a] device for cooling a semiconductor component comprising a chamber enclosing a non-metallic crystal forming material which experiences a phase change at a temperature which corresponds to the desired operating temperature of the semiconductor component and wherein the material is in thermal communication with the semiconductor component.” U.S. Pat. No. 3,780,356). However, U.S. Pat. No. 3,780,356 discloses only the use of hydrates of the inorganic salts of alkali or alkali earth metals, eutectics of the chlorides, nitrates, acetates, or ammoniates of the light metals.
See also, for example, U.S. Pat. No. 5,508,884 (disclosing a “[s]ystem for dissipating heat energy generated by an electronic component and sealed enclosure used in a system of this kind. The system for dissipating heat of the invention is of the type comprising a sealed metal enclosure the interior volume of which contains a fluid, said component being fixed to the outside wall of said enclosure and connected to a heat dissipator by said enclosure and is characterized in that the phase change temperature of the fluid at atmospheric pressure is less than a given maximum ambient temperature and in that said enclosure also contains a volume of gas different from the fluid so that variation in the ambient temperature causes a variation in the pressure inside the enclosure which modifies the phase change temperature of the fluid.” U.S. Pat. No. 5,508,884). However, the invention of U.S. Pat. No. 5,508,884 does not disclose the use of any material other than phase change liquid.
It is apparent from the aforementioned prior art that there is no known way to provide an efficient and sustainable means of sufficient heat removal from electronic devices without burdening the design with increased power consumption or bulk. It is therefore desirable that electronic device cooling technology provide cooling which is efficient, in that it does not require any power consumption, and capable of dissipating large enough quantities of heat to allow the electronic device to achieve optimum performance while adding only minimal bulk to the design.